Emulsion inks are well known as the inks used in stencil printing systems and inkjet printing systems or the like.
A stencil printing system is a system in which a stencil printing base paper is used to form a printing plate, and printing is then conducted onto a printing target such as a sheet of paper by causing an ink to pass through the perforations formed in the base paper during plate preparation. Because of its favorable operability and relative simplicity, stencil printing is used in a wide range of fields.
An inkjet printing system is a printing system in which printing is conducted by spraying a liquid ink with a high degree of fluidity from very fine nozzles, and adhering that ink to a recording medium such as a sheet of paper. This type of system enables the printing of high-resolution, high-quality images at high speed and with minimal noise, using a comparatively inexpensive printing apparatus, and is rapidly becoming widespread.
Water-in-oil (W/O) emulsion inks are generally used as the emulsion inks. When a water-in-oil emulsion ink is printed onto a printing target such as a sheet of paper, first, the oil phase component(s) that constitute the outer phase of the emulsion penetrate into the paper or the like, and subsequently, the water phase component(s) that constitute the inner phase penetrate into the interior of the paper or the like and/or are scattered on the paper or the like. In the case of a water-in-oil emulsion ink that includes a coloring component within the oil phase, the coloring component penetrates into the paper or the like together with the oil phase, and as a result, the coloring component penetrates into the interior of the paper or the like, increasing the likelihood of show-through. Further, this penetration phenomenon also occurs at the surface of the paper, which can sometimes cause the ink to spread across the paper surface, causing bleeding.
In order to address these types of problems, Patent Document 1 proposes a water-in-oil emulsion ink in which the colorant is incorporated within the water phase. With this type of emulsion ink, because the oil phase first penetrates into the paper or the like, and the water phase then penetrates thereafter, the coloring component within the water phase can be prevented by the oil phase from penetrating too deeply into the paper or the like. As a result, an image quality can be obtained that exhibits a high print density, and minimal ink bleeding and show-through.
Patent Document 2 provides a water-in-oil emulsion ink in which by blending a water phase containing a pigment into the oil phase, stirring the resulting system, and then continuously adding a water phase containing a water-soluble polymer to the system under constant stirring, the water phase can be dispersed uniformly while the production line is simplified, yielding a water-in-oil emulsion ink having a print density similar to conventional inks.
Patent Document 3 proposes a method of addressing the problems of print dot size and image density that are common to both aqueous inks and oil-based inks, by using a water-in-oil emulsion ink.
[Patent Document 1] Japanese Patent Laid-Open No. H11-1649
[Patent Document 2] Japanese Patent Laid-Open No. H10-168370
[Patent Document 3] Japanese Patent Laid-Open No. 2006-56931
Generally, in order to ensure that the gloss of the printed image is similar to that of the printing target such as a sheet of paper, which enables better discernment of the text or the like of the printed item, a matte printed image of low gloss is preferred. However, if a conventional emulsion ink is used, then the printed image tends to cause specular reflection of external light, meaning the images of text or the like can sometimes be difficult to distinguish.
In Patent Document 2, water phases containing different components are added separately and continuously to the oil phase and subsequently emulsified, and because, in this manner, the water phases dispersed within the oil phase are combined and the components contained therein are dispersed equally during the emulsification process, the water phase within the final ink has an overall uniform composition. In other words, although the production method differs, the ink actually has the same configuration as conventional emulsion inks, and suffers from the same problems outlined above.